The standard Carnot reversible heat pump cycle, as simplified by use of a throttling valve for expansion of the refrigerant fluid and a mechanical compressor for compression of the fluid, has been in use for a wide variety of applications for heating and for cooling for many years and is so well-known as to require little explanation. It should be sufficient for the purposes of this application to explain that in the cooling mode, such systems pass saturated liquid refrigerant through an expansion valve to a lower pressure. This is a constant enthalpy process and the temperature of the refrigerant falls to the saturation temperature at the final pressure because of the latent heat demanded by the vaporization. The refrigerant is then passed through an evaporator wherein heat is absorbed from the atmosphere or from some other medium which it is desired to cool. The refrigerant vapor is then drawn into a compressor and compressed to a temperature at which a cooling medium, which may be ambient air, can condense the refrigerant vapor as it passes through a condenser. The cycle is frequently reversible so that when the system operates as a heat pump, with the energy added to the system being supplied by the compressor and by the ambient air, it is used to supply heat to a flow system such as the interior of a building. The present invention relates to improvements and simplifications of the above described system in which a heat source, as an energy supply, is substituted for the mechanical compressor now in widespread use as an energy source.